Three-dimensional development of luminal projections and junctional complexes in the developing central nervous system blood vessels of the rat.

The luminal surface features and junctional complexes from developing blood vessels in the rat central nervous system have been studied by high-voltage electron microscopy and scanning electron microscopy. Developing blood vessels exhibit three types of luminal projections; marginal folds or ridges at junctional complexes, ridges not at junctional complexes and microvilli. Both types of ridges are associated with troughs or depressions in the luminal surface of the endothelial cell. Those ridges not associated with junctional complexes take part inthe production of enclosed tunnels in the endothelial cell cytoplasm. Fusion of the external leaflets of junctional complexes between adjacent endothelial cells occurred, initially, near the luminal surface of the blood vemina secondarily. Further fusion activity to produce the zonula occludens type junction appeared to spread outwards from the smaller fusion sites.